The life cycle of plants is highly regulated at the genetic level and very sensitive to environmental variables, such as day length and temperature (Koomneef et al. (1998) Annu. Rev. Plant. Physiol. Mol. Biol., 49: 345-370). Plant life cycles are complex and must be regulated to maximize chances of survival. There must be a careful balance of investment of time in various stages of the life cycle of a plant, such as dormancy, germination, reproduction, vegetative growth and maturation, to insure survival. To implement this balance, each of these stages requires the expression or repression of various genes or groups of genes.
The life cycle of flowering plants in general can be divided into three growth phases: vegetative, reproductive, and seed development. In the vegetative phase, the shoot apical meristem (SAM) generates leaves that later will ensure the resources necessary to produce fertile offspring. Upon receiving the appropriate environmental and developmental signals the plant switches to floral, or reproductive, growth and the SAM enters the inflorescence phase and gives rise to an inflorescence with flower primordia. During this phase the fate of the SAM and the secondary shoots that arise in the axils of the leaves is determined by a set of meristem identity genes, some of which prevent and some of which promote the development of floral meristems. Once the floral organs are produced and fruits are formed, the plant enters seed development phase (Xu et al. (1995) Plant Mol. Biol. 27.237). If the appropriate environmental and developmental signals that induce the plant to switch to floral, or reproductive, growth are disrupted, the plant will not be able to enter reproductive growth, and will maintain vegetative growth.
Temporal coordination of life cycle stages depends on factors such as energy requirements, environmental variables and reproductive strategy. Maximization of any one life cycle period or stage is usually at the cost of one of the other periods or stages in the life cycle. For instance, trade-offs exist between the initiation of the reproductive growth stage and the length of the vegetative growth stage, the early flowering and the later growth and reproduction stages. The transition of a plant from a vegetative growth stage into a reproductive, or flowering, stage is pivotal because the fitness of a plant, and therefore its chances of survival, can be highly sensitive to this transition.
Thus, early flowering in plants can provide discernable advantages over other later flowering plants. For instance, early flowering usually results in early maturity and eventually shortens growth duration from sowing to harvest in plants. This allows farmers to avoid environmental adversity, such as freezing temperatures, in the early or later growing seasons at high latitude or altitude. This competitive advantage can also help crop rotation schedules. Alternatively, if cold weather or crop rotation is not a problem, later flowering has other advantages, such as a robust vegetative state, yielding higher amounts of plant material.
These advantages to the plant also translate into economic and production advantages providing more efficient human use of plants and especially plant crops critical to human survival. Generally, early flowering plants have smaller plant stature due to shortened vegetative growth compared to wild-type plants. Small plant stature is desirable in some cases. However, technologies enabling alteration of the moment within the life cycle at which a plant flowers are more advantageous if this alteration is accomplished without other disadvantageous trade-offs, such as a reduction in plant mass or other changes in phenotypically discernable traits.
Availability and maintenance of a reproducible stream of food to feed people has been a high priority throughout the history of human civilization. Specialists and researchers in the fields of crop science, horticulture and forest science are even today constantly striving to find and produce plants with an increased growth potential to feed an increasing world population and to guarantee a supply of reproducible raw materials. The robust level of research in these fields of science indicates the level of importance leaders in every geographic environment and climate around the world place on providing sustainable sources of food for their people.
Manipulation of crop performance has been accomplished conventionally for centuries through selection and plant breeding. The breeding process is, however, both time-consuming and labor-intensive. Furthermore, appropriate breeding programs must be specially designed for each relevant plant species.
On the other hand, great progress has been made in using molecular genetics approaches to manipulate plants to provide better crops. Through introduction and expression of recombinant nucleic acid molecules in plants, researchers are now poised to provide people with plant species tailored to grow more efficiently and produce more product despite unique geographic and/or climatic environments. These new approaches have the additional advantage of not being limited to one plant species, but instead being applicable to multiple different plant species (Zhang et al. (2004) Plant Physiol. 135:615).
Despite this progress, today there continues to be a great need for generally applicable processes that improve forest or agricultural plant growth to suit particular needs depending on specific environmental conditions. To this end, the present invention is directed to advantageously manipulating plant life cycles to maximize the benefits of various crops depending on the benefit sought and the particular environment in which the crop must grow, characterized by expression of recombinant DNA molecules in plants. These molecules may be from the plant itself, and simply expressed at a higher or lower level, or the molecules may be from different plant species, providing advantages from one species to another allowing the new plant species to better adapt to a challenging geographic or climatic environment.